FIG. 1 illustrates a prior art auxiliary power unit 10 of the type manufactured by the assignee of the present invention. The power unit 10 has a plurality of electrical and mechanical elements located at distributed locations on the outside surface 11 of the power unit. The power unit has a plurality of electrical communication lines 35, 38 and 40 which interconnect an electronic control unit 12, a condition monitor panel 14, sensors 16-20 and fuel control 24. A conventional gas turbine engine (not illustrated) is located within the envelope defined by the outside surface 11.
FIG. 2 illustrates a prior art control system for a turbine engine of a power unit of the type illustrated in FIG. 1 and manufactured by the assignee of the present invention for use as an auxiliary power unit (APU), integrated power unit (IPU), and/or an emergency power unit (EPU). Like reference numerals in FIGS. 1 and 2 identify like parts. A turbine engine (not illustrated) has a plurality of electrical and mechanical control elements mounted at distributed locations within or on the exterior surface of the power unit. The engine has numerous electrical communication lines connecting the distributed elements as described below. As a consequence of the turbine engine being utilized in airframe applications, overall weight is significant in the operational efficiency of the airframe. Additionally, the overall size of the turbine engine including electrical and mechanical controls is significant in achieving efficient space utilization in an airframe.
Poor space utilization, increased expense and increased weight are a consequence of the distribution of the control system as illustrated in FIGS. 1 and 2. The electronic control assembly 12 containing a programmed microprocessor is interfaced with the condition monitor panel 14. The condition monitor panel 14 contains a plurality of visual indicators 15 which signal the status of built-in test equipment (BITE), an hourmeter 17 and a start counter 19. The condition monitor panel 14 is interfaced with a plurality of sensors which include a speed pickup 16, an oil pressure sensor 18, a pair of thermocouples 20, respectively sensing exhaust gas temperature (EGT) at different locations in the exhaust gas stream produced by the turbine, and an oil temperature sensor 22. Fuel control assembly 24 contains a plurality of fuel control valves. A start control valve 26 controls the flow of fuel for a starting sequence used to start the turbine engine. A main control fuel valve 28 controls the flow of fuel for controlling the operation of the turbine engine. A return fuel control valve 30 controls the return of fuel to the fuel tank (not illustrated). A fuel inlet 32 couples fuel from the fuel tank to the aforementioned valves 26-30. A fuel outlet 33 couples fuel back to the fuel tank. Operation of the fuel control is conventional. The electronic control assembly 12 and condition monitor panel 14 are wired to receive electrical power applied from either a battery or an electrical power generator (not illustrated). Electrical power is applied to the electronic control assembly 12 and condition monitor panel 14 by electrical power line 34 connected to the battery or electrical power generator.
A significant expense in the overall cost of the turbine engine and a reliability problem are the electrical communication lines connecting the distributed elements together and the overvoltage protectors and high frequency filters which are connected to the electrical communication lines at the point of entry to the electronic control 12. A plurality of electrical communication lines interconnect the electronic control assembly 12, condition monitor panel 14, sensors 16-22 and fuel control valves 28-30. A multiple wire communication line 35 connects the electronic control assembly 12 and the condition monitor panel 14. An overvoltage protector 36 is coupled to each one of the individual communication lines 35 to protect the electronic control assembly 12 from high voltage conditions caused by EMP or a lightning strike which could produce a high electrical voltage that could damage or destroy the electronic control assembly. Communication lines 38 couple the sensors 16-22 to the condition monitor panel 14. Communication lines 40 couple control signals produced by a control program stored in the electronic control assembly 12 to the valves 26-30 of the fuel control assembly 24 to control the fuel flow to the turbine engine. A filter assembly 41 comprised of an inductor and a pair of capacitors shunting first and second terminals of the inductor to ground is connected in series with each of the communication lines 35 to filter high frequency noise (EMI or RFI) to ground originating from diverse locations in the power unit such as the elements 16-22. The overvoltage protectors 36 and the filter assembly 41 are conventionally mounted within the EMI tight housing containing the electronic control 12 which adds appreciable weight. Furthermore, the overvoltage protectors and filters 41 within the housing of the electronic control 12 may cost up to 30% of the overall cost of the power unit.
Each of the valves 26, 28 and 30 is driven by a power switch (not illustrated) within the electronic control 12. The power switch driving each of the valves 26, 28 and 30 has an overload protection circuit which turns off the power switch when a short circuit exists in the solenoid control of the valves. Short circuits producing an overload on the switches can be caused by damage to exposed communication lines 40. The overload protection circuit adds weight, may represent up to 5% of the overall cost of the power unit and adds complexity with attendant reliability problems.
As illustrated in FIG. 1, the electronic control assembly 12, condition monitor panel 14, sensors 16-22 and fuel control 24 are physically mounted at different locations on the outside surface 11 of the power unit 10. As a consequence of the distribution of these elements on different locations of the power unit 10, the necessary electrical communication lines and fuel lines add weight to the power unit. Additionally, mounting of the aforementioned components on the exterior surface 11 of the power unit is complicated in that it is difficult to find surfaces well suited for the mounting of these parts which do not interfere with the operation of the engine and do not add substantial size to the outside envelope of the power unit. Finally, a reduction of the distributed components greatly reduces the number of line replaceable units. Each of the elements 14-22 are line replaceable units. Specifications regarding the prior art require built-in test equipment inside the electronic control 14 for each line replaceable unit which adds complexity, weight and cost.